The invention relates to a combined earth-star sensor system for three-axis attitude determination of satellites in space, as well as a method for determining the orbit and attitude of space vehicles.
For earth-oriented satellites, infrared sensors are presently used to determine roll and pitch angles, while solar sensors are used to determine the yaw angle of the satellite. This solution is associated with the disadvantage of the arrangement of several solar sensor heads used being very expensive, and of the lack of any yaw reference in the case of co-linearity of the earth and the sun. Furthermore, evaluation additionally requires an exact knowledge of the attitude of the satellite on its orbit. Moreover, errors between the measuring axes of solar sensors and earth sensors, e.g. caused by inaccuracies during installation or as a result of thermal influence, can significantly degrade the quality of measurements.
Known star sensors provide a three-axis attitude reference in an inertial system. Here too, earth alignment requires an exact knowledge of the satellite attitude. At present, CCDe are used for such star sensors. Due to the their lack of radiation resistance, they are not suitable for use in orbits with substantial radiation load, e.g. for use in geosynchronous missions.
From EP 0 589 387 A1 a combined earth-star sensor system is known which uses a single optical arrangement to carry out both star observation and earth observation in the UV range. To carry out common observation, the optical arrangement comprises a wide-angle system incorporating a mirror system. Due to the coinciding directions of observation for earth observation and star observation, the known system is only able to observe starts in proximity to the earth; and due to the sensor being designed with the strong earth radiation in mind, for attitude determination the known system is only able to observe stars of high intensity.
FIG. U.S. Pat. No. 5,189,295 a combined earth-star sensor system for three-axis attitude determination in space is known, with the combined earth-star sensor system comprising separate apertures with different directions of observation and common image pickup device for the earth sensor and the star sensor. The earth is partially shielded by a screen, so as to allow acquisition of the pole star. There is however a problem that the difference in brightness between the earth and stars has not been taken into account adequately. Consequently, adequate acquisition of the earth and stars is not ensured.